Electric power steering (EPS) is a direct replacement for hydraulic power steering, but consumes significantly less energy during operation. Hydraulic power steering uses a mechanical torque sensor to measure the torque applied to a steering wheel by a driver. The torque sensor is coupled to a valve, which in turn directs hydraulic fluid so as to amplify the applied torque. EPS works in a similar manner, however an electronic torque sensor is used to measure the steering wheel torque, and an electric motor is used to apply additional torque to the steering rack. Failure of either system can make the vehicle relatively difficult to steer.
EPS capability may be especially attractive when used aboard a hybrid vehicle having an internal combustion engine and one or more electric traction motors. Hybrid vehicles typically run in an electric vehicle (EV) mode up to a threshold vehicle speed. The engine is off in the EV mode(s). Therefore, mechanical power which is ordinarily provided by engine torque is discontinued, which precludes the use of a conventional hydraulic power steering system using an engine-driven fluid pump.